1. Field of the Invention
The present invention relates to semiconductor devices, and more particularly, to a semiconductor device including a transistor which has a wide range of safe power operation and which operates in a stable manner.
2. Description of the Background Art
In a conventional power transistor of this kind included in a semiconductor device, a plurality of unit transistors are formed on the same semiconductor substrate. The collectors, bases, and emitters of respective unit transistors are electrically connected in parallel to each other, so as to enlarge area of safety operation of the power transistor.
FIG. 3 is a plan view illustrating overall interconnections of the power transistor formed of conventional unit transistors.
Referring to FIG. 3, the power transistor includes a plurality of conventional unit transistors connected in parallel.
FIG. 4 is an enlarged view of a portion A in FIG. 3.
Referring to FIG. 4, portion A shows a plan view of conventional unit transistors forming the power transistor shown in FIG. 3, and includes an epitaxial layer (collector region) 1 of a first conductivity type (N type), a base region 2 of a second conductivity type (P type), an emitter region 3 of the first conductivity type, a base contact portion 4, and an emitter contact portion 5.
FIG. 5 is a view showing the correspondence between a two-dimensional structure and a cross sectional structure of the conventional unit transistors forming the power transistor.
FIG. 5(a) shows a two-dimensional structure of the conventional unit transistors, (b) shows a cross sectional structure of the conventional unit transistors taken along the line A-A' in (a), and (c) shows a cross sectional structure of the conventional unit transistors taken along the line B-B' in (a).
Referring to FIG. 5(a), the two-dimensional structure of the conventional unit transistors includes epitaxial layer 1 of the first conductivity type, base region 2 of the second conductivity type, emitter region 3 of the first conductivity type, base contact portion 4, and emitter contact portion 5.
Referring to FIG. 5(b), the cross sectional structure of the conventional unit transistors includes epitaxial layer 1 of the first conductivity type, base region 2 of the second conductivity type, emitter region 3 of the first conductivity type, a buried layer 6 of the first conductivity type, and a semiconductor substrate 7 of the second conductivity type.
Referring to FIG. 5(c), the cross sectional structure of conventional unit transistors includes epitaxial layer 1 of the first conductivity type, base region 2 of the second conductivity type, emitter region 3 of the first conductivity type, buried layer 6 of the first conductivity type, and semiconductor substrate 7 of the second conductivity type.
In addition, an oxide film (SiO.sub.2, for example) 8 and a metal electrode (Al, for example) 9 are shown in FIGS. 5(b) and (c).
Referring to FIG. 5(a), epitaxial layer 1 of the first conductivity type is a collector region common to respective unit transistors, and in the collector region, base region 2 of the first conductivity type is formed, in which the comb-shaped emitter region 3 of the second conductivity type is formed. Base contact portion 4 is provided on a "tooth" of the comb-shaped base region 2, and unit transistors are connected to each other by metal interconnections, not shown. Emitter contact portions 5 are provided in the central region and on a tooth of the comb-shaped emitter region 3, and metal interconnections (not shown) are formed. At the contact portion of the collector region which is formed of epitaxial layer 1 and common to respective unit transistors, a contact common to respective unit transistors is provided and metal interconnections are formed, which are not shown in FIG. 5.
In the specification, epitaxial layer 1 of the first conductivity type will also be referred to as collector region 1 of the first conductivity type hereinafter.
Referring to FIG. 5(b), a base resistor region 2a (pinch resistor region) serving as a base resistor R.sub.B of a transistor having the emitter contact portion 5 as an emitter is formed in base region 2. Base resistor R.sub.B has a resistance value substantially determined by pinch resistance. A length L2 shown in FIG. 5(b) is the length of base resistor region 2a.
Referring to FIG. 5(c), emitter resistor region 3b, serving as an emitter resistor R.sub.E of a transistor having an emitter active region 3a as an emitter, extends in emitter region 3 on both sides of central emitter contact portion 5. Emitter active region 3a serving as an emitter of a unit transistor is formed on one side of emitter resistor region 3b. A length L1 shown in FIG. 5(c) is a length of emitter resistor region 3b.
FIG. 6 is an equivalent circuit diagram of the unit transistors in such power transistor.
Referring to FIG. 6, the equivalent circuit includes transistors Tr and Tr', base resistor R.sub.B, and emitter resistor R.sub.E.
Respective collectors of transistors Tr and Tr' are connected to each other. Base resistor R.sub.B has one end connected to the gate of transistor Tr and the other end connected to the gate of transistor Tr'. Emitter resistor R.sub.E has one end connected to the emitter of transistor Tr and the other end connected to the emitter of transistor Tr'.
Portions D1-D6 shown in FIG. 5 correspond to D1-D6 in FIG. 6, respectively. More specifically, emitter active region 3a shown in FIG. 5(c) corresponds to the emitter of unit transistor Tr in FIG. 6 (D4). Emitter active region 3a is connected to metal interconnection 9 (D1) formed on emitter contact portion 5 common to respective unit transistors and located at the central region of FIG. 5 through emitter resistor region 3b (corresponding to emitter resistor R.sub.E) (D5). Emitter contact portion 5 in the central region of FIG. 5(c) corresponds to the emitter of unit transistor Tr' in FIG. 6 (D1). The region of emitter contact portion 5 is connected to a metal interconnection formed on base contact portion 4 through base region 2 located immediately under emitter contact portion 5 and base resistor region 2a (or base resistor R.sub.B) shown in FIG. 5(b) (D2). Epitaxial layer (collector region) 1 shown in FIG. 5 corresponds to the collector of unit transistor Tr (D6), and base region 2 adjacent to an inner end of epitaxial layer 1 corresponds to the gate of unit transistor Tr (D3).
Such a power transistor is designed to operate in a stable manner, avoiding a concentration of current on one particular unit transistor. For this purpose, it is known that each unit transistor should be designed so that the following relation of the equation (1) is established (Japanese Patent Publication No. 5-76768): EQU R.sub.B =h.sub.FE /A.times.R.sub.E (1)
Here, R.sub.B represents the resistance value of the base resistor (pinch resistor) connecting respective bases of the unit transistors to each other, h.sub.FE represents the current amplification ratio of the unit transistor, A represents the ratio between plane areas of the emitter contact portion 5 and emitter active region 3a, and R.sub.E is the resistance value of the emitter resistor.
Power transistors are required to have a wide area of safety operation, and in order to expand such area the resistance value of emitter resistor R.sub.E of each unit transistor has been adjusted to be greater and naturally the resistance value of base resistor R.sub.B has also been adjusted to be greater for the sake of stable transistor operations as can be seen from the equation (1).
However, lengths L1 and L2 of emitter resistor region 3a and base resistor region 2a, respectively, must be made longer as shown in FIGS. 5(b) and (c) so as to adjust resistance values of emitter resistor R.sub.E and base resistor R.sub.B of the unit transistor as shown in FIG. 5 to be greater.
Description will be made with specific values. Assuming that current amplification ratio h.sub.FE of the unit transistor is 200, ratio A between plane areas of the emitter contact portion 5 and emitter active region 3a is 1, and emitter resistor R.sub.E has the resistance value of 10 .OMEGA., then 2 k.OMEGA. is required as the resistance value of base resistor R.sub.B as can be seen from the equation (1). Assuming, under such conditions, that sheet resistance R.sub.ES of the emitter of the unit transistor is 6 .OMEGA./.quadrature., a width W1 of the emitter resistor region is 10 .mu.m, sheet resistance R.sub.BS Of the pinch resistor corresponding to base resistor R.sub.B is 6 k.OMEGA./.quadrature., and a width W2 of the base resistor region is 35 .mu.m, then length L1 of the emitter resistor region will be 16.7 .mu.m and length L2 of the base resistor region will be 11.7 .mu.m to yield: L1+L2=28.4 .mu.m. However, if the resistance value of emitter resistor R.sub.E is changed from 10 .OMEGA. to 12 .OMEGA. to expand the area of safety operation of the power transistor, base resistor R.sub.B is required to have the resistance value of 2.4 k.OMEGA. as obtained from the equation (1). Accordingly, length L1 of the emitter resistor region will be 20 .mu.m, and length L2 of the base resistor region will be 14 .mu.m to yield: L1+L2=34 .mu.m. As a result, the space for the sum (L1+L2) of lengths L1 and L2 of the emitter resistor region and the base resistor region of the unit transistor is undesirably increased.
As described above, if the area of safety operation of a power transistor is to be expanded and the stable operation thereof is to be obtained, the unit transistor is increased in size, leading to an increase in size of the entire semiconductor device including the power transistor.